When launching an EC2 server instance or creating a volume within AWS, it is fairly straightforward to have the volumes be encrypted. It's simply a checkbox. However, if you decide after creating an unencrypted volume that you want it to be encrypted, you can't simply toggle the "encrypted flag" for the volume. This article explains how to encrypt the volumes for an entire server.

While this article is for an entire server, if you need to encrypt just a single volume, there is a similar process that can be done at the volume-level.